This invention relates to a method of and apparatus for treating a gas stream containing hydrogen sulphide.
Gas streams comprising hydrogen sulphide are formed, for example, as waste gases in oil refinery or gas refinery operation. In view of their hydrogen sulphide content, these gas streams cannot be discharged to the atmosphere without first being treated so as to remove almost all the hydrogen sulphide.
A standard method of treating such gas stream serves to recover sulphur there from is by the Claus process. Conventional Claus processes are described in the introductory paragraphs of EP-A-565 316.
WO-A-00/10693 discloses with reference to its FIG. 1 a Claus process employing two catalytic Claus stages. Downstream of the second catalytic Claus stage the effluent gas is subjected to catalytic reduction to form a resulting gas stream containing sulphur vapour and hydrogen sulphide. The sulphur vapour is extracted by condensation to leave a residual gas mixture containing 0.5 to 1.2% by volume of hydrogen sulphide. The residual gas mixture may be treated by any one of a number of different methods, including extraction of its water vapour content by condensation and catalytic oxidation of the residual gas. Sulphur formed as a result of the catalytic oxidation step is extracted by condensation.
EP-A-565 316 also discloses a process in which in a first reactor a part of the hydrogen sulphide content of a feed stream comprising hydrogen sulphide is oxidised to sulphur dioxide, and thus formed sulphur dioxide is reacted with residual hydrogen sulphide to form sulphur vapour and water vapour. A partially reacted gas stream including sulphur vapour, water vapour, residual hydrogen sulphide and residual sulphur dioxide is withdrawn from the furnace. A sulphur condenser is employed to extract sulphur vapour from the partially treated gas stream so as to form a sulphur vapour depleted gas stream. At least part of the sulphur depleted gas stream is sent to a catalytic reactor in which its sulphur dioxide content is reduced by hydrogen to hydrogen sulphide. Water vapour is extracted from the resulting reduced gas stream. The water vapour-depleted gas stream is then preferably recycled to the furnace. A purge stream is taken from a chosen position in the above described cycle and is subjected to further treatment so as to render it fit for discharge to the environment. The purpose of the recycle is to obtain a very high effective conversion of hydrogen sulphide to sulphur vapour in the furnace and thereby facilitate the attainment of a total conversion efficiency which is sufficient to meet any prevailing environmental standard.
By using pure oxygen (or air highly enriched in oxygen) the size of the initial furnace may be kept down. However, the advantages in size reduction of the initial furnace made possible by the use of pure oxygen (or oxygen highly enriched in air) as the oxidant are counteracted to some extent by the recycle of gas to the furnace. Although EP-A-0 565 316 further discloses that the recycle can be omitted, this is stated not to be preferred as it has an adverse effect on the effective percentage conversion of hydrogen sulphide to sulphur in the furnace.
One solution to this problem suggested in EP-A-565 316 is to employ an amine separation unit to concentrate the recycle stream in hydrogen sulphide. Such amine separation units, however, tend to be particularly costly, even if only of a small size.
It is an aim of the method according to the invention to provide an alternative solution to this problem which does not necessitate a recycle.